WASHINGTON — Counterterrorism officials are chasing a credible but unconfirmed tip that al Qaeda has plans to set off a car bomb in New York City or Washington around the Sept. 11 anniversary, with bridges or tunnels as potential targets. It was the first word of a possible “active plot” timed to coincide with commemoration of the terror group’s attacks a decade ago.

Police in New York and Washington increased their already stepped-up staffing levels. In New York City, authorities were stopping vehicles at the 59th Street bridge Friday, causing a major traffic backup. National Guard troops and transit police carrying assault rifles watched the crowds at Penn Station.

White House officials said President Barack Obama had no plans to change his scheduled trips to New York’s ground zero, the Pentagon and Shanksville, Pa., on Sunday to mark the anniversary.

This latest threat “should not surprise any of us,” Secretary of State Hillary Rodham Clinton said during a speech in New York. Telling the public “is intended to enlist the millions and millions of New Yorkers and Americans to be the eyes and the ears of vigilance,” she said.

Law enforcement officials were pursuing three people who may be traveling to the U.S. or who have recently entered the country, based on the detailed information received by the U.S. intelligence community late Wednesday, officials said.

Senior U.S. counterterrorism officials said Friday that authorities have a general sense of who is behind the plot and who may be tasked to carry it out, but they don’t have exact identities yet. They said there is at least one U.S. citizen among the three people.

The intelligence suggested that al Qaeda planned to car bomb one of the two cities that were hit 10 years ago, in coordinated attacks that killed nearly 3,000 people.

Vice President Joe Biden said Friday that there was no confirmation that anyone had traveled into the U.S. for such a plot although the tip came from a credible source. “There’s no certitude,” he said.

“The thing we are all most worried about is what they call a ‘lone ranger,’ a lone actor, not some extremely complicated plan like it took to take down the World Trade towers,” said Biden, who appeared on the trio of network morning TV shows Friday.

U.S. officials said the source of the terror tip indicated that al Qaeda’s new leader, Ayman al-Zawahri, was involved in planning the plot. But the official also said that many in the intelligence community question that and other aspects of the source’s information.

The nation’s terror alert level has not changed, although raising it was under consideration Thursday night.

At the Pentagon, officials said there have been no changes to military base security levels since they were upgraded earlier in the week, before the threat information came in. And there have been no changes to the schedules of Defense Secretary Leon Panetta and Joint Chiefs Chairman Adm. Mike Mullen, who will be attending 9/11 commemoration events throughout the weekend.

The officials described the threat to the Associated Press on condition of anonymity because they were not authorized to publicly discuss sensitive security matters.

Security has been enhanced around the country in the weeks leading up to the 10th anniversary, a date officials have long known could draw an attack. Law enforcement officials have been particularly wary after information gleaned from Osama bin Laden’s compound in Pakistan indicated that al Qaeda had considered attacking the U.S. on this anniversary and other important American dates. Officials have also been concerned that terrorists would see the anniversary as an opportunity to retaliate against the U.S. for killing bin Laden in a military raid in May.

Officials said that so far they have no reason to believe that there is a direct connection between this new threat and the information found in the compound laying out al Qaeda’s aspirational goals.

The FBI and Homeland Security Department issued a joint intelligence bulletin Thursday night to law enforcement around the country urging authorities to maintain increased security and be on the lookout for suspicious activity.

The threat came in a single piece of information and was so specific — and came at such a time of already heightened alert — that it could not be ignored, officials said.

“There is specific, credible but unconfirmed threat information,” the head of the FBI’s New York division, Janice Fedarcyk, said. “As we always do before important dates like the anniversary of 9/11, we will undoubtedly get more reporting in the coming days.”

New York Mayor Michael Bloomberg told reporters that police there were deploying additional resources around the city but that New Yorkers should go about their business as usual, and the city’s observance of the attacks will go on as planned.

In Washington, law enforcement officials said they were working 12-hour days indefinitely, and Police Chief Cathy Lanier said unattended cars parked in unusual locations risked being towed.

Law enforcement officials are checking out all of the details included in the threat, Rep. Peter King, R-N.Y., said.

“No need to panic,” King said. “They have not been able to confirm it yet.”

Intelligence officials had not seen any specific or credible threats regarding an attack around the anniversary before Wednesday.

• Associated Press writers Matt Apuzzo, Lolita C. Baldor, Julie Pace and Eric Tucker in Washington and Tom Hays in New York contributed to this report.